The present invention relates to an oxide superconducting wire, and particularly to an oxide superconducting wire with reduced a.c. loss.
Generally, a superconducting multi-filamentary wire follows the critical state model, Wxe2x88x9dH3 (H less than Hp), and Wxe2x88x9dH (H greater than Hp), where W represents loss and H represents applied magnetic field. Hp represents full penetration field, and Hp=xc2x7d/2, where d represents size of a filament and Jc represents critical current density. The same phenomenon also has been confirmed in oxide superconductors and metal superconductors. The loss is called hysteresis loss and is produced by irreversible movements of magnetic flux in the proximity of pinning potential. If the magnetic flux lines move only within the pinning potential, their movements become reversible. Hence, the loss mentioned above is not produced and the a.c. loss of the wire is expected to be significantly reduced.
This phenomenon has been confirmed in a metal superconducting wire having sub micron size filaments. It has been confirmed in the metal superconducting wire, that when the size of the filament is equal to or less than xcex0xe2x80x2 (penetration depth of Campbell""s a.c. magnetic field), the a.c. loss characteristics cannot be explained according to the critical state model, and the amount of a.c. loss drops significantly below the amount of loss expected from the critical current model.
The same effect is expected to hold also in an oxide superconducting multi-filamentary wire, although it has not been confirmed in experiments due to the theoretical imperfection of the pinning phenomenon in oxide superconductors.
For a metal-coated oxide superconducting wire, the number of filaments has been increased mainly to improve its mechanical strength. The shape and the size of the filament, however, have not been optimized in view of the a.c. loss reduction, because basic studies of the a.c. characteristics of the oxide superconducting wire is yet at a fledging stage and little has been known about it.
In addition, though it has recently come to be generally accepted that the behavior of oxide superconductor at a.c. follows the critical state model qualitatively, various research groups are still investigating the a.c. loss characteristics in the oxide superconducting wire and more detailed quantitative interpretation thereof has not been acquired.
An object of the present invention is to provide a low-loss oxide superconducting wire utilizing a reversible phenomenon.
An oxide superconducting wire according to the present invention, formed of a metal coated multi-filamentary superconductor containing a plurality of superconducting filaments fit in a matrix, is characterized in that when the wire is electrified with an a.c. current in liquid nitrogen (77 K),
dxe2x89xa62xe2x80x83xe2x80x83(1),
where d represents size of the superconducting filament in xcexcm.
An oxide superconducting wire according to the present invention, formed of a metal coated multi-filamentary superconductor containing a plurality of superconducting filaments fit in a matrix, is characterized in that when the wire is electrified with an a.c. current in liquid nitrogen (77 K),
dxe2x89xa62xe2x80x83xe2x80x83(1),
and
d/(2xcex0xe2x80x2)xe2x89xa61xe2x80x83xe2x80x83(2),
where d represents size of the superconducting filament in xcexcm, and xcex0xe2x80x2 represents penetration depth of Campbell""s a.c. magnetic field.
An oxide superconducting wire according to the present invention, formed of a metal coated multi-filamentary superconductor containing a plurality of superconducting filaments fit in a matrix, is characterized in that when the wire is electrified with an a.c. current at a temperature t (K) lower than the critical temperature of the multi-filamentary superconductor,
dxe2x89xa6{76/(107xe2x88x92t)}xe2x88x920.4xe2x80x83xe2x80x83(3),
where d represents size of the superconducting filament.
In the present invention, when the superconducting filament is of tape-like shape, the superconducting filament size d represents an average thickness of the tape-like filament.
In the present invention, Bi-2223 phase, for example, is used for the oxide superconductor.
Further, in the present invention, a silver alloy, for example, is used as a metal coating material.
Still further, in the present invention, the superconducting filament may be divided into plural portions to be multi-filamentary.
In the present invention, the matrix is preferably of an alloy with a higher resistance than silver. When such an alloy is used for the matrix, the loss can be reduced.
Still further, in the present invention, the multi-filamentary superconducting filament is twisted, because twisting can further reduce the loss.